Surface morphology evolution mechanisms of pulse laser polishing mold steel

In this study, high-frequency nanosecond pulse laser was conducted on mold steel surfaces for solving surface defects during polishing processes. The influence of different single-pulse energy densities on surface roughness was carefully investigated through technical experiments, with the lowest roughness obtained at 2.5 J/cm2. A computational fluid dynamics (CFD) model was developed to simulate the evolution of surface morphology at different energy densities. The applicability of the model was successfully verified by comparative experiments and simulative results. The evaporation threshold for nanosecond pulse laser polishing was found to be between 2.5 and 3.0 J/cm2. The introduction of evaporation and increased temperature gradients accelerate the flow of the melt pool, thereby leading to a shift in the driving force for fluid velocity. The short pulse cycle can create a continuous melt pool that moves with the laser beam, resulting in surface flattening along the scanning path. Under a large number of overlapping pulses, the surface over melting (SOM) mechanism dominated the morphology evolution of the molten pool. Significant findings demonstrated that pulse laser-induced heat accumulated to melt the peaks and valleys of the initial surface, while convective flow reduced the height difference of the melt pool. It can provide a new insight into surface roughness improvement of metals through nanosecond pulse laser polishing.